Thermoplastic film drum data storage equipment



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United States Patent O 3,195,112 THERMUPLASTEC FLM DRUM DATA STORAGE EQUlPh/IENT Wiiliam C. Hughes, Scotia, and .iohn E. Wolfe, Schenectady, NSY., assigner-s to General Electric Company, a corporation of New York Original application Aug. 25, 1958, Ser. No. 757,983. Divided and this application Mar. 12, 1%3, Ser. No. 264,571

Claims. (Cl. 340-173) The present invention relates to a new and improved drum data storage equipment having uinque addressing features, and is a division of copending US. application Serial No. 757,083, filed August 25, 1958, and now abandoned, entitled Thermoplastic Film Data Storage Equipment, W. C. Hughes and J. E. Wolfe, inventors, assigned to the General Electric Company, the assignee of the present invention.

More particularly, the invention relates to a new and improved drum data storage equipment that employs a solid impressionable thermoplastic film recording medium upon which data is recorded. The equipment is capable of storing relatively large quantities of data in a comparatively small space, and can be read out in response to a request for any particular block of data in comparatively short access times.

A copending U.S. patent application Serial No. 756,775 Thermoplastic Film Data Storage Equipment, filed August 25, 1958 and now abandoned, W. C. Hughes, l. Wolfe and W. E. Glenn, inventors, assigned to the General Electric Company, describes a new and improved data recorder employing a thermoplastic film recording medium. The present invention describes and claims improved features of construction for a drum thermoplastic film data recorder of the type disclosed in the above identified application Serial No. 756,775, and which constitutes a practical working arrangement of the data recorder equipment generally disclosed and claimed in that application.

To be more particular, it is a primary object of the present invention to provide a new and improved drum thermoplastic film data recorder which employs a rotatable and axially translatable drum, having a solid impressionable thermoplastic film recording medium secured to its surface. The drum is constructed in such a manner that it can be readily rotated and translated in either direction axially to provide quick access to any desired block of data recorded thereon in response to an address supplied thereto.

Another object of the invention is to provide a new and improved data recorder employing a solid impressionable thermoplastic film recording medium which has unique addressing characteristics.

In practicing the invention, a new and improved solid impressionable thermoplastic film data recorder is provided which comprises a rotatable drum having a thermoplastic film recording medium formed on the surface thereof. The drum is both rotatable and translatable in either direction along its axis in response to an address. An electron writing apparatus is positioned adjacent to the drum for impressing electrons on the solid impressionable thermoplastic film recording medium in desired intelligence conveying patterns representing the data to be stored. Heating means are provided for heating the surface of the impressionable solid thermoplastic recording medium to a substantially liquid state to cause deformations of the surface in accordance with the electron patterns. The deformations thus recorded are then preserved upon the surface of the solid impressionabl thermoplastic film recording medium being returned to a solid state by cooling. Readout means are provided for inspecting the impressionable medium after the light modifying deformations are formed thereon to derive output electrical signals indicative of the intelligence contained in the electron patterns. In a preferred form of the invention, the readout means comprises an optical assembly for projecting an optical image of the intelligence patterns formed on the impressionable recording medium, and electron-optical means for deriving output electric signals representative of the intelligence formed in the patterns. In a further preferred embodiment of the invention, color separating means are interposed between the optical assembly and the electron-optical means with the electronoptical means viewing the projected optical image through the color separating means. The new and improved data recorder further includes automatic positioning means for positioning the rotatable drum both rotationally and longitudinally in any desired location with respect to the electron writing apparatus, the heating means, or the readout means, Position control means are provided for controlling the operation of the automatic positioning means which comprises a first selsyn system for accurately rotating the drum to position the thermoplastic film recording medium in a first dimension, and a second selsyn system for accurately moving the drum in either direction along its longitudinal axis to position the thermoplastic recording medium in a second dimension substantially at right angles with respect to the first dimension. Where the new and improved data recorder is to be employed as a digital data recorder, a digital-toanalog converter is employed for controlling each of the selsyn systems of the position control means in response to digital type address control signals. Also, in preferred arrangements of the new and improved thermoplastic film data recorder, readout logic circuit means are provided which employ data block edge recognition means to assure the fidelity of readout of the data stored. If desired, an electrically operative memory device may be employed in conjunction with the recorder. In further arrangements of the new and improved solid impressionable thermoplastic film data recorder, a program control circuit means is incorporated in the equipment for selectively controlling the operation of at least the electron writing apparatus, the position control means, the heating means, and the readout means.

Other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIG. l is a functional block diagram showing the general arrangement of a drum data storage equipment constructed in accordance with the present invention;

FIG. 2 is a combined functional block diagram and circuit diagram of a servo positioning system used with the drum data storage equipment of FIG. l;

FIG. 3 is a functional block diagram of the writing system used in the drum data storage equipment of FIG. 1;

FIG. 4 is a circuit diagram showing the details of construction of the horizontal deflection control circuits used in the writing system of FIG. 3;

FIG. 5 is a functional block diagram showing the construction of the heating head control circuit comprising a part of the drum data storage equipment shown in FIG. l;

FIG. 6 is a circuit diagram showing the details of construction of the vertical deflection circuits used in the writing system of FIG. 3;

FIG. 7 is .a functional bloclr diagram of the reading System comprising a part of the drum data storage equipment shown in FIG. 1;

FIG. 8 is a fragmentary View of the thermoplastic film surface of the storage drum comprising a part of the drum data storage equipment shown in FlG. 1, and illustrates the manner in which data is written on the drum surface;

Y FIG. 9 is a functional block diagram of the parts f a readout system used in the drum data storage equipment of FIG. 1; l

FIG. 10 is a circuit diagram showing the master clock and frequency dividing circuits of the reading system shown in FIG. 7;

FIG. 11 is a circuit diagram of a video amplifier comprising a part of the reading system shown in FlG. 7;

FIG. 12 is a circuit diagram of the horizontal sweep matching circuits comprising a part of the reading system shown in FIG. 7;

FIG. 13 is a circuit diagram of the vertical sweep matching circuits of the reading system shown in FIG. 7;

FlG. 14 is a circuit diagram of the auxiliary power supply circuits for the readout vidicons which comprise a part of the reading system shown in FlG. 7;

FIG. 15 is a circuit diagram of the output logic circuits comprising a part of the reading system shown in FIG. 7;

FIG. 16 is a circuit diagram of the trace rejection logic circuits comprising a part of the reading system Vshown in FIG. 7;

FIG. 17 is a combined functional block diagram and circuit diagram showing the construction of the core shift ,register unit which comprises a part ot the drum data Storage equipment of FIG. 1; Y

FIG. 18 is a functional block diagram showing the construction of the program control unit that is used in the drum data storage equipment shown in FlG. 1; and

FIG. 19 shows a series of potential waveforms of the characteristics of the signals appearing at various points in the data storage equipment of FG. 1.

DRUM DATA STORAGE EQUlFlt/IENT General block diagram The embodiment of the invention illustrated in FlG. 1 of the drawings includes an automatic positioning means for automatically positioning a thermoplastic lrn recording medium in response to a control signal, and comprises a drum storage unit wherein thermoplastic tllm is formed'over the surface of the drum, and the data to be stored is formed by impressions made on the surface of the thermoplastic film. In this embodiment of the invention a rotatable transparent drum l1 is provided which has a thermoplastic iilm surface secured thereto. The thermoplastic nlm may be formed from a blend of polystyrene, m-terephenyl and a copolymer ot 95 weight percent of butadiene and 5 weight percent styrene as disclosed in copending US. patent application Serial Number 8,842, tiled February 15, 1960 and now Patent No. 3,113,179, entitled Method, Apparatus and Medium for Recording, William E. Glenn, lr., inventor, which is a continuation in part of US. patent application Serial Number 698,167, iiled November 22, 1957 and now abandoned, William E. Glenn, Ir., inventor, entitled Method and Apparatus for Electronic Recording, assigned to the General Electric Company. The rotatable drum Ill is journaled in a rack 123, and has one end of it-s shaft secured to a rotational servo drive motor 13, and the remaining end of the shaft secured to selsyn generator 14. The rack 112 has a ratcheted extension 15 on one of the ends thereof which engages and is driven by a second servo motor 16 that serves to drive the rack 12 longitudially along the axis of the rotatable drum 11 to any desired position. Also coupled to the rack 12 ythrough a suitable gear arrangement, not shown,

i is a second selsyn generator 17. By this arrangement, the rotational servo motor 13 serves to rotate the drum 1l to any desired position along its periphery and the selsyn generator ld will generate an electric signal indicative of this position. The longitudinal Vservo motor lo serves to position the drum l1 to any desired position along its longitudinal axis, and the second selsyn generator 17 serves to develop an electric signal indicative or the longitudinal position of the drum. Both the rotational drive motor 13 and irst selsyn generator 14 are included in a lirst selsyn system No. 1 for accurately positionin.o the drum 11 rotationally, and both longitudinal drive motor lo and second selsyn generator 17 are included in a second :selsyn system for accurately -positioning the drum 1l longitudinally. Because both of the selsyn systems are identical in construction, only one of the systems has been disclosed at the right hand side of the drawing.

ln addition to the motor 13 and selsyn generator 14, the rst selsyn system comprises an address register 19 electrically coupled to a digital-analog converter 2b which converts the digital type electric address signal supplied to the address register from a computer with which the equipment'is being used, indicated at Z1, to an analog type electric signal which in turn is applied to the selsyn generator 1d to accurately position servo motor 13 in response to the address. The selsyn generator 114i is coupled back to the servo motor 13 through a suitable servo amplilier 22 and actuating switch 23 to form a closed loop selsyn system. To further assure accuracy in positioning of the drum 1l, it is usually desirable to couple the motor 13 through reduction gears indicated at 24 to the drum, and to provide a braking arrangement indicated at 25 for the motor 13. Because the selsyn system will be described in greater detail subsequently, a further description of its construction and operation is believed unnecessary at this point. lt should be indicated, however, that the rst selsyn system serves to accurately position drum ill at a desired point rotationally in response to an address supplied to the address register 19 thereof from computer Z1. The second selsyn system, not shown, would include the motor lo and second selsyn generator 17, and would serve to position the drum 11i longitudinally at a desired position in response to an address supplied thereto from computer 2l. n Y ln order to form impressions on the thermoplastic surface of the drum 11, an electron beam Writing apparatus is provided which includes an electron optics device 2s which provides a pencil-like writing electron beam that can be scanned across the surface of the thermoplastic iilm on the drum 1l .in the view of therlevices, and to impress electron charges upon the surface of the thermoplastic iilm in desired patterns. The construction and operation of the electron optics device is described more fully in U.S. application S.N. 757,081, Thermoplastic information Storage System7 by S. P. Newberry and J. F. Norton, General Electric Patent Docket 14E-1335, tiled concurrently hercwith, and assigned to the General Electric Company. However, it may be Well to point out that the electron optics device 23 does serve to impress upon the thermoplastic lilm surface of drum l1 small light diffraction gratings of two different grating spacings for diiiiracting light of two dilierent colors through a selective aperture. These gratings, or bits as referred to hereinafter, are formed 'by the electron beam from the electron optics and comprise -a number or series of parallel lines which are spaced apart a distance determined by the color desired to be retracted or reilected. To provide gratings capable of diiTracting light of two different colors, it would be necessary to provide two dierent grating types characterized by different `spacings between the lines making up the gratings. Spacing ot the electron beam marks in each grating is controlled by a writing optics control circuit 27 which is electrically connectedto the deflection .and accelerating electrodes of the electron optics device to control its operation. The write optims control circuit 24 is in turn controlled by control signals supplied there to from a program controller 28 and from a working core shift register 29. The core `shift register 29 is designed -to accommodate a block of information in digital form which would represent some 32 by 32 gratings or bits of information, as hereinafter referred to. This digital information is stored in the core shift register for convenience, and is read out serially and supplied to the write optics control 24 through a :suitable connection indicated at 31. The information contained in the shift regi-ster 29 would be originally supplied thereto from the computer 2li.

Also positioned adjacent the rotatable drum 11 is a read out means which in this instance comprises a light source 33 located within transp-arent drum 11 and positioned to illuminate a particular portion of the surface of the drum 11 so that light is refracted through the gratings thereon lsuch that a a particular diffraction angle two different colored Arays emerge from the illuminated portion as deter- -mined by the grating spacing. Gratings previously formed on the surface of the drum 11 diffract the light from light source 33 in two different distinctive colors, one of Wh-ich represents a Zero (O) in the binary code, and the other a one (l). The different colored light images are transmitted through a light optic system 34 to a color separating 'filter arrangement 35 and 36. A half-silvered mirror 37 serves to separate the light rays into two sepayrate images, and direct the two images through the respective color filters 35 and 36 to an associated electron camera tube 38 or 39 which may be of the vidicon type. Electric signals generated by each of the vidicons 38 and 39 are supplied to an output logic circuit d1 which selectively supplies the data read out from drum 11 to the Working core shift register 29 in the form of electric pulses represent-ing the information in digital form. It follows, of course, that when the equipment is being used to read out information stored on the drum 11, the working Ishift register 29 is used to store the information supplied thereto by the output logic circuit. This information may then be used by the computer as required in any computation operation in which the computer may be involved. The operation of the vidicons 33 land 39 in reading out information supplied thereto by the optical assembly 34 is controlled by a deiiection circuit 42 which in turn is controlled from the programmer 2S in response to commands received from the computer.

Prior to writing the electron patterns on the thermoplastic film to form the diffraction gratings, it may be necessary to heat the surface of the film to condition it to receive the charge patterns. Also, it may be desired to replace data already recorded on the thermoplastic lm of the drum 11 with corrected data supplied to the core `shift register 29 from the computer 2l. For both of these purposes, and for curing the charge patterns as will be described hereinafter, a heating head d3 is mounted on the end of the electron optics device 26 adjacent the surface of the drum 11 as described in the above referred to patent application of Newberry and Norton. The heating head 43 is connected to a heating control circuit 4d that in turn is controlled from the programmer 28. The heating head 43 serves to apply heat to the thermoplastic film on the surface of drum 11 to condition it to cure it or to erase data already recorded thereon. Because of the nature 'of the thermoplastic film surface on the drum il, sufiicient heat for a critical period of time will serve to erase the gratings formed on the thermoplastic film. The new information in the core shift register may then be written 4on the thermoplastic film in the place of the 4old information that was erased.

As can be readily appreciated, the equipment can be used either to write data on the drum 1li to store the same for an indefinite period of usage latter, or alternatively to read out information that has been stored on the drum for use in some computational operation by the computer.

Assuming it is desired to record some data on the surface of the drum ll, then the computer 32 supplies appropriate command signals to programmer 23 which in turn will initiate operation of the electron writing apparatus 26 through the Write optics control circuit 27. Simultaneously, the data to be stored is supplied serially from core shift register 29 to the write control circuit after having been previously pla-ced in the working core shift register 29 from the computer 21. This data is in digital form, and accordingly may be stored on the surface of the d-rum il by a series of two distinguishable gratings or bits. The nature of these bits will be described more fully hereinafter together With a full description of the manner in which bits are formed on the Isurface of the drum l. lt is believed adequate at this point in the disclosure to mention that the `bits are in the form of a series of several equal length parallel lines, with one set of the lines representing for example Zero (0) being spaced apart at a given distance ranging from 0.5 to 5 microns, and the other set -of lines representing `for example one (l) being spaced apart a different distance within the Same range. It can be appreciated, therefore, that a long line 0f bits of information can be made which would represent binary data in the form of a series of ones and zeros. Having impressed the electron charges on the drum 11 in the desi-red patterns, with certain types of thermoplastic film it may be desirable to cure the charges by placing them under the heating head 43 for a short period of time in order to facilitate the action of the charge in forming the small marks or lines on the thermoplastic film. In this eventuality, after completion of recording of a block of bits of information contained in the working shift register 29, the programmer 23 may be set to actuate the heating .head control circuit i4 thereby supplying heating current to the heating head 43, and effect curing of the block of bits by heating them thereby causing them to become viscose or fluid, at which point the charge will deform the film to produ-ce the desired gratings. The programmer may then rotate the drum 11 under the electron optics device, yand if need be advance it longitudinally to position it to accept the next block of bits to be recorded thereon. In order ythat any particular block of bits may be readily identified for the information contained therein, it is, of course, necessary that the data being recorded by properly coded by the location of the block in which it is written, or the coded identification be placed in an easily accessible position on the block of bits, or by some other means.

In order to utilize the equipment to read out information previously stored on the drum 11, it is merely necessary that the computer 21, or some other similar addressing mechanism, supply the desired address to the address register 19 in each of the selsyn systems driving the rotational servo motor 13, and the longitudinal servo motor 16. The two selsyn systems will then accurately Vposition the drum 1l under the read out optical system 34 in response to the command from the computer 21 to the programmer 28. On being properly located, each of the selsyn systems will supply a signal to the programmer 28 indicating that the drum 11 is properly located Whereupon the programmer will actuate the deflection circuits ft2 of the read out vidicons 33 and 39, as well as the light source 33. The vidicons 38 and 39 then function to read out the information stored in the block of bits through the output logic circuit 41 to the Working core shift register 29. Thereafter the information may be used by the computer 32 in its operations as required.

From a consideration of the above brief description of the new and improved data storage equipment, it can be appreciated that applicants have provided an entirely new data storage device capable of attaining high storage density thereby rendering possible the construction of a very small data storage equipment containing a large amount of stored information. Further, it can be appreciated that the device is highly reliable in operation,

a relay tree to be described more fully hereinafter.

and is capable of providing desired information in a minimum access time.

Position servo mechanism FlG, 2 of the drawings discloses the details of the construction of the seisyn systems No. 1 and No. 2, and in view of the fact that both systems are identical in construction, although they perform different functions, it is believed necessary to disclose only one of the systems in detail. The selsyn system is designed to drive the drum Vllll to a desired position with respect to either the read out means or the electron beam writing apparatus. As was explained in connection with the systems shown in FiG. l of the drawings, the drum ll is geared to servo motor 13 which may comprise any conventional servo motor such as those described in chapter 9 of the textbook entitled Servo Mechanism Practice by William R. Ahrendt, published by McGraw-Hill Publishing Cornpany, i954. Also mechanically connected to drum 11 so as to be rotated with lit is the rotor winding 54 of the selsyn generator 14, and the rotor 56 of a rate generator to be described more fully hereinafter. To get the servo motor 13 to position the drum lll in a desired location with respect to electron beam writing apparatus, yor the read out means of the data storage equipment, a source of reference potential, indicated at l, is connected across the statorL field winding S2. Servo motor 13 is of the conventional type wherein torque will be developed depending upon the magnitude and polarity of the energizing signal supplied to the first field winding 52, and a second field winding 53 which is positioned traversely to the first field winding 52 so as to develop a rotating torque. As iswell known with motors of this type, if an electro-magnetic flux is supplied from only one of the windings, for example field winding 52, the rotor of the motor ll will not rotate, but will rotate only if there is an angular relationship between the electro-magnetic flux supplied from the two field windings 52 and 53. the field supplied by field winding 52 is held steady, then the direction of rotation as well as the magnitude of l' will develop an error voltage whose magnitude and polarity is dependent upon the mechanical poistion of the rotor winding 50.- with respect to the electromagnetic flux generated in stator winding 53 by the control potential. This error voltage is then ampliiied and fed to servo motor 13 and field winding S3 to cause motor 13 to rotate rotor winding 5d to zero out the error voltage.

The error voltage developed across rotor winding 54 is tapped off a load resistor 55 and coupled to a second load resistor 53 which is connected across one of the field windings 59 of rate generator 56. The load resistor Se is Vin turn connected to the movable contact of still a third load resistor 6l connected across a secondaryV winding 62 of a transformer whose primary winding is connected to The load resistor (il is connected directly to the control grid of an electron tube 63 which is a conventional resistance-capacitance coupled power amplifier which amplifies the error signal supplied from resistor 61, and couples the amplified error signal through a resistance-capacitance coupling network to the control grid of an electron tube 64. Electron tube 64, in conjunction with an electron tube 65, and a voltage dividing resistor interconnecting the plates of electron tubes olland 65, and having a mid-tap point grounded, comprise a phase splitting network. p This phase splitting network serves to develop two opposed in vphase electric signals from the single signal supplied thereto from amplifier 63, and to couple the two signals to the control grid of a push-pull power amplifier comprised by electron tubes 67 and 6?. The electron tubes 67 and have their anodes connected to opposite ends of a 'primary winding 69 that is inductively coupled to a secondary winding lil which in turn is connected across the second field winding S3 of servo motor 13. Also connected across the secondary winding 7i is a solenoid winding 7E of a relay having its movable contacts 74 connected in series circuit relationship with a sourceof alterhating current voltage and the field winding '75' of a normally closed brake 25. By this arrangement, the appearance of a voltage across the secondary winding 71 is applied not only to the second field winding 53 of servo motor i3, but also serves to actuate relay '74, which in turn releases the brake 25 to allow the motor 13 to move to a new position in response to the error voltage applied to its second field winding 53.

The position that the rotor of servo motor i3 takes, and hence the position of the drumY lll, is dependent upon the magnitude and polarity of the electric signal supplied across the second field winding 53 of Vservo motor i3 by the servo amplifier described inthe preceding paragraph. it is possible by controlling this electric signal tocontrolthe position of the drum 1l. This control is achieved by controlling the nature of the exciting voltage supplied across the primary or stator windings 5@ of selsyn generator ld. Control of exciting voltage is in turn achieved by the cross bar switches and 8o, and the remaining address mechanism for actuating Vthe cross bar switches to be described hereinafter.

The mechanism for actuating cross b ar switches 3S and titi comprises a plurality of bistable multivibrators or flip flop amplifiers '75, whcih are connected in series circuit relationship, and which have .an address data input source Vconnected to the rst one thereof. Also, a source of shift signal pulses from the master clock oscillator of the equip- 'ment is connected in parallel circuit relationship to the inputs of all of the fiip flop amplifiers for effectively shifting data set into the flip flop amplifiers from the address data source to the next succeeding flip flop amplifier in the string until the entire register is filled with the desired address data. The last fiip iiop amplifier '75 in the address register has its output connected to the programmer, and 1s designed to receive data indicating whether the equip- `ment is to read or to write. For example, the output of the flip flop amplifier may be represented by a Zero (G) voltage or a voltage level representing a one (l) in binary form, and it may be chosen that the read indication from flip flop 77o is a one (l), while the write indicationwill be Vrepresented by a zero (0). Accordingly, liip `flop amplifier 76 will provide to the programm-er a signal indicating whether the equipment is to read or to write. Simultaneously, this signal is supplied through a conductor 77 to the control grid of an ampliiier 73 having a solenoid winding i9 connected in the plate cir-cuit thereof for operating a selector switch Si), the function of which will be described more fully hereinafter.

The first two groups of four flip flop amplitiers 75 are connected through two pyramid arrays of diode rectifiers Y till and S2 which comprise diode switches. Each of the diode switches Si and 32 comprise conventional pyramid diode matrices of the type described on page 4l of the ,textbook entitled Digital Computer Components and Circuits by R. K. Richards, published by D. Van Nostrand Company, 1957, and function to selectively connect desired ones of the outputs of the group of four flip flop ampliliers 75 to which they are connected, to the input of associated output ampliers S3 and 84, respectively. Each of the output amplifiers 83 and Se `are connected to respective associated holding magnet solenoid windings S7 and Siti, respectively, which serve to actuate holding magnets 

1. IN A DIGITAL DATA INFORMATION STORAGE DEVICE THE COMBINATION OF A ROTATABLE DRUM HAVING AN IMPRESSIONABLE SOLID THERMOPLASTIC RECORDING MEDIUM SECURED TO THE SURFACE THEREOF, A READOUT DEVICE FOR INSPECTING DISCRETE AREAS OF THE THERMOPLASTIC RECORDING MEDIUM AND DERIVING AN OUTPUT ELECTRIC SIGNAL INDICATIVE OF THE INTELLIGENCE IMPRESSED ON THE THERMOSPLASIC RECORDING MEDIUM BY ELECTRON WRITING, AUTOMATIC POSITIONING MEANS FOR ACCURATELY POSITIONING SAID DRUM ROTATIONALLY AND LONGITUDINALLY IN ANY DESIRED LOCATION WITH RESPECT TO SAID READOUT DEVICE, POSITION CONTROL MEANS FOR ACCURATELY CONTROLLING THE OPERATION OF SAID POSITIONING MEANS, AND READOUT LOGICAL CIRCUIT MEANS OPERATIVELY COUPLED TO SAID READOUT DEVICE FOR CONTROLLING THE OPERATION OF THE READOUT DEVICE IN ACCORDANCE WITH A PRESET SCHEDULE, SAID READOUT LOGICAL CIRCUIT MEANS INCLUDING DATA BLOCK EDGE RECOGNITION MEANS FOR IDENTIFYING THE BEGINNING EDGE OF A DATA BLOCK DURING READOUT. 